From 59ce465f6d51f1d3b8ae18a3346264006e99aab5 Mon Sep 17 00:00:00 2001
From: Florian Fischer <florian.fischer@muhq.space>
Date: Sun, 8 Jun 2025 11:09:30 -0500
Subject: fix the line number detection with tabs

If a line includes tabs add the visually added space for each tab stop
to the line length in order to detect a wrapped line.
---
 init.lua      | 11 ++++++++---
 util.lua      | 25 +++++++++++++++++++++++++
 util_test.lua | 15 +++++++++++++++
 3 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/init.lua b/init.lua
index fd8c0b4..498887d 100644
--- a/init.lua
+++ b/init.lua
@@ -566,9 +566,14 @@ local function file_lineno_to_viewport_lineno(win, file_lineno)
     local view_lineno = 0
     for n = win.viewport.lines.start, file_lineno do
       view_lineno = view_lineno + 1
-      -- Wrapped line this shifts our displayed line down
-      if #win.file.lines[n] > win.viewport.width then
-        view_lineno = view_lineno + math.floor(#win.file.lines[n] / win.viewport.width)
+      -- Wrapped lines shift our displayed line down
+      local line_len = #win.file.lines[n]
+      if not win.options.expandtab then
+        line_len = util.visual_chars_in_line(win, win.file.lines[n], line_len)
+      end
+
+      if line_len >= win.viewport.width then
+        view_lineno = view_lineno + math.floor(line_len / win.viewport.width)
       end
     end
     return view_lineno
diff --git a/util.lua b/util.lua
index 04855fb..2b090b2 100644
--- a/util.lua
+++ b/util.lua
@@ -279,4 +279,29 @@ function util.vis_pos_to_sel(win, pos)
   return sel
 end
 
+--- Count the visual characters in a line
+-- This is useful to detect wrapped lines including tabs which may add a
+-- unspecified amount of white space to a line depending on their position and
+-- the used tabwidth.
+-- @param win the window containing the line
+-- @param line the line to count
+-- @param nchars the number of characters in the line
+-- @return the number of visual characters
+util.visual_chars_in_line = function(win, line, nchars)
+  -- fast string iteration inspired by:
+  -- https://stackoverflow.com/a/49222705
+  local l = {string.byte(line, 1, nchars)}
+  local line_len = 0
+  for i = 1, nchars do
+    local c = l[i] -- Note: produces char codes instead of chars.
+    if c == 9 then -- '\t'
+      local chars_to_tab_stop = win.options.tabwidth - (line_len % win.options.tabwidth)
+      line_len = line_len + chars_to_tab_stop
+    else
+      line_len = line_len + 1
+    end
+  end
+  return line_len
+end
+
 return util
diff --git a/util_test.lua b/util_test.lua
index fd6570d..a0f4218 100755
--- a/util_test.lua
+++ b/util_test.lua
@@ -15,4 +15,19 @@ function test_dirname() -- luacheck: ignore 111
   lunatest.assert_equal('/', util.dirname('/'))
 end
 
+function test_visual_chars_in_line() -- luacheck: ignore 111
+  local win = {options = {tabwidth = 4}} -- win mock
+  local s = '\tfo' -- visual chars == 6
+  lunatest.assert_equal(util.visual_chars_in_line(win, s, #s), 6)
+
+  s = 'f\tfo' -- visual chars == 6
+  lunatest.assert_equal(util.visual_chars_in_line(win, s, #s), 6)
+
+  s = 'fo\tfo' -- visual chars == 6
+  lunatest.assert_equal(util.visual_chars_in_line(win, s, #s), 6)
+
+  s = 'foo\tfo' -- visual chars == 6
+  lunatest.assert_equal(util.visual_chars_in_line(win, s, #s), 6)
+end
+
 lunatest.run()
-- 
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